Historians leafing through Britain's official government papers in decades to come could easily skip past Wednesday, 27 June 2007 without realising what a momentous day it was. 'PM answers PMQs and resigns. New PM to the Palace,' is all the Downing Street 'grid' says of the departure of Labour's most successful prime minister after a decade in office.

The 'grid', which marks every government movement in fine detail, is hardly gushing in its description of what will happen the following day. 'New PM's first day,' is the headline for Thursday, when Gordon Brown will chair his first cabinet meeting as prime minister.

The dry, understated language belies months of intensive preparation by Brown, who is determined to kick off his premiership with a dramatic blitz to mark a departure from the Blair era.

'I feel we've got to meet new challenges; that means new priorities,' Brown says of how he will turn over a new leaf. 'But the new priorities can only properly be met by a new form of politics and a new way of governing.'

He will face a nervous wait on Wednesday lunchtime before he finally gets his hands on power. Buckingham Palace has let it known that the Queen, ever a stickler for constitutional tradition, will be calling the shots.

Sir Robin Janvrin, her private secretary, has passed on a message that the man who is destined to become Her Majesty's eleventh Prime Minister should sit by the telephone to await his summons. Only when Tony Blair has left the palace - once again a humble backbench Labour MP - after tendering his resignation will the phone in Brown's office ring with news that he should make the short car journey down the Mall.

The constitutional proprieties, with the Queen presiding over the change of power, mean there will be no Blair/Brown handshake on the steps of No 10. The closest Britain will see to a public handover will come tomorrow when Blair attends the special conference in Manchester where Brown will be officially anointed as his successor as the new leader of the Labour party. Blair, who once toyed with the idea of removing his old friend from the Treasury at the height of their feuding, will offer his unqualified support for Brown in an explicit public declaration that he will not follow the example of Margaret Thatcher's 'back-seat driving'.

'Tony will go to the palace and then he'll be off,' one No 10 source said of the Prime Minister, who will retreat to the seclusion of Chequers while builders put the finishing touches to his grand Connaught Square house.

Power is already slipping away for Blair, as evidenced by last week's European Union summit in Brussels. In a series of telephone calls on Friday morning, Brown to all intents and purposes assumed control of Britain's negotiations when he warned Blair not to accept a key French change to the new EU treaty. A fresh protocol, to run alongside the treaty, was hurriedly drawn up to signal that the EU would still be committed to 'undistorted competition', a core declaration at the heart of the community for 50 years and described by the Tories as one of the union's 'crown jewels'.

Brown's dramatic intervention showed his confidence - and his determination to ensure a smooth path in his first months in office - as he finally prepares to take over the reins of power. Aides say they have never seen the Chancellor more relaxed, though he knows he faces a daunting task if he is to repeat the Tories' trick of winning the fourth election in a row for his party despite a change of leadership. The question now is, how will he do?

'This isa spectacular event,' a beaming Brown whispered to aides on Wednesday evening as he stepped out of the lift at the Mansion House to attend his eleventh and final annual Bankers' and Merchants' Dinner as Chancellor. Dressed in his trademark lounge suit, with his faithful adviser and ministerial colleague Ed Balls a few paces behind, Brown walked past an honour guard into a private room, where he was greeted by the great and the good of the City.

A week before his move into No 10, Brown was out to enjoy himself, a far cry from the initial wariness of the Chancellor who has always refused to don the traditional black tie for the Mansion House dinner. 'I am just going to write one envelope [with advice] for my successor,' he told his black tie audience. 'When you give the Mansion House speech, wear a black tie.'

The audience laughed as Brown finally acknowledged that his dour approach to their banquet may have been slightly misplaced. 'Brown used to come here and machine gun us with dreary speeches,' one senior financier said. 'At last he seems to be opening up and relaxing a bit.'

Some habits never die, though. Brown spurned the wine - a white Pouilly Fume Domaine des Berthiers 2004 and a red Chateau Branaire Ducru Saint-Julien 4eme Cru 1996 - to toast the Queen with a glass of water. But bibulous city figures were still impressed. 'Brown makes David Cameron and George Osborne look like a boy band,' one senior financier said.

Such remarks will be relished by the Brown camp, who know their man will never ape what he has dismissively called the 'celebrity' of the Blair era. But they hope that an earnest Brown, more humble and relaxed, will appeal to voters, a point which may be borne out by today's Ipsos Mori poll which shows him enjoying an 18 point lead over David Cameron on the question of who is best placed to be Prime Minister.

Brown knows he has just a few weeks to convince the electorate that he can signal a departure from the Blair era, without vacating the political centre ground occupied so successfully by New Labour in the past decade. Unlike Blair, whose freshness in 1997 gave him an extended honeymoon, Brown knows he has a narrow window because he is such an established figure.

A flurry of announcements, which have been prepared in minute detail over the past months, will signal the new style. The new cabinet will mark a symbolic break; Brown is said to be keen to recruit John Denham into a senior government position. The highly respected chairman of the Commons home affairs select committee resigned without fanfare from the government on the eve of the Iraq war.

Brown is still determined to include figures from outside his party in his government, though he is struggling on this front after a botched attempt to recruit the former Liberal Democrat leader Paddy Ashdown. An attempt to secure the services of John Stevens, the former Metropolitan Police Commissioner, as a minister also failed, though Stevens is expected to take a role as an adviser.

Outsiders will still have a role in a cross-party convention that will take charge of constitutional reforms. Brown is said to have been eyeing up centre-ground Tories who were involved in the Britain in Europe group. One fan of the present Prime Minister described the cross-party initiative as 'win-win' for Brown. 'If he wins over people from other parties that shows a new, open approach,' the MP said. 'If he fails then at least he tried.'

Others detected a whiff of the old, scheming Brown. 'This was a classic Brownite plot,' one Liberal Democrat frontbencher said. 'He went behind Ming's back and has left us feeling very uncomfortable. This will rebound on Brown.'

The grand neo-classical splendour of Lancaster House, constructed in distinctive Bath stone on the Mall in the early nineteenth century, has played host to major political events, most notably the 1979 talks that led to the independence of Zimbabwe. On Thursday afternoon, as London basked in muggy weather on midsummer's day, a rather more low key event took place as Brown attempted to show a new side.

With a notepad on his knee, Brown spent three hours walking from table to table in the Long Gallery, calmly taking notes as environmentalists debated how to save the planet at a climate change conference. 'Can I join you for a second,' Brown asked as he approached each table during breaks between speeches at the event, which was addressed by Al Gore.

Twiddling his pen, Brown would look up from his notes to ask questions in interventions that ranged from the pointed to the humorous. 'So what is the solution, you need to define the solution,' he told one table.

The sight of the Prime Minister in waiting with a notepad on his knee was designed to illustrate one of the key themes of his premiership: that he has abandoned his 'top-down' approach to politics and is ready to listen. One aide insisted the change was genuine.

'Gordon used to be bored at events like this. But now he is engaged. We said to him politics has changed. You cannot be Blair Mark II. He understands that and he realises that he must engage in this way. I was not sure whether he would enjoy it but he really does seem to have been energised by it.'

Brown himself says he is a convert. 'It is liberating to go round the country and listen to what people are saying and to feel that you can act on what they are saying on a broader canvas. But at the same time try to play a part in bringing people together.'

The Lancaster House event will serve as a model for one of the big ideas of the Brown premiership: citizens' juries where ordinary people, rather than experts, will have a direct input into policy. Brown, who held a citizens' jury event yesterday, is determined to avoid a repeat of Labour's notorious 'Big Conversation' of a few years ago, which was seen as a cover for imposing unwelcome ideas on the party.

A new form of 'citizens' jury service' will be drawn up to formalise the process and to involve people from all levels of society, though crucially this will not be compulsory.

'By citizens' jury service I mean inviting the public in the same way that they are invited to serve on a jury to give us some of their insight into how we can do things better,' he says.

Sam Brereton, 30, from Harlow in Essex, who was born with Down's Syndrome, has an infectious smile. At a drinks reception for carers in No 11 Downing Street on a sweltering evening the week before last, Sam was the star of the evening as he grinned and joked his way round the room.

When Brown entered No 11's State Room on the first floor Sam rushed up to the Chancellor who, without missing a beat, hugged him. 'Sam is very tactile - he hurled himself at Gordon Brown who responded very naturally,' Sam's mother, Mary, said.

The Brown camp hope this private side of Brown will come across to the nation. But they know they have their work cut out because Brown lacks the natural ease Blair enjoyed on the most important medium of all: television. Replacing Brown's awkward TV face with the more natural smile seen in small gatherings may be his key challenge in the run-up to the next election.

Brown will take to the stage in Manchester this afternoon with today's poll showing the first signs of a 'Brown bounce' and jitters among Tories. When he addresses his party for the first time as leader Brown will know he must move quickly and decisively to ensure he builds a sustained lead and enjoys more than a brief honeymoon with the electorate.

TB The pleading hands - Blair's trademark gesticulation was open-palmed, reaching out to the audience, as if begging for its approval

GB The clunking fist - Brown was thus described by Blair in the Commons.

TB Keepie-uppie - Blair, never shy of a photo opportunity, liked to feign an interest in football to demonstrate his common touch.

GB The full body tackle. Brown was a talented rugby player before injury put a stop to his sport.

TB High Church - Blair is an Anglican, reported to be about to turn to Catholicism. Big on ceremony.

GB Low Church - Brown is the son of a Presbyterian minister. Big on austerity.

TB 'Sofa government' - Blair was criticised in the Butler Report for conducting government business too informally.

GB 'Government of all the talents' - Brown claims he will eschew tribalism when making key appointments.

TB The cheesy grin - Blair beams from ear to ear.

GB The poker face - Brown glowers from brow to jaw.

TB Zeitgeist - Blair's greatest natural political gift was an uncanny knack of surfing the cultural mood of the times.

GB Schadenfreude - Brown has a reputation for relishing the downfall of political rivals.

TB The Millennium Dome - extravagant folly in east London, meant to symbolise Britain's exuberant culture of youthful energy and creativity, while also regenerating benighted corner of the capital. It ran hopelessly over budget and caused endless embarrassment for the government.